New York ? All diamonds start out the same — as colorless crystalized carbon. What’s done to make them bright, brilliant, desirable and valuable, though, is as varied as the people who wear them.

Even the giant gems that end up at a jeweler such as Harry Winston have subtle differences: Some are “emerald-cut,” others are “cushion-cut.” And don’t forget about the round-cut diamond, which was the standard engagement ring of a previous generation.

There are also pear-shaped diamonds, hearts, baguettes and marquises.

The name of the diamond literally describes its shape, explains Mary Ann Mognaz, a sales executive at Harry Winston.

A cushion-cut diamond has the appearance of a pillow; a round-brilliant is, well, round and brilliant. “Cushion cuts are popular with the younger set. It’s an old cut that’s been revived. It has the brilliance of a round diamond but has a little something extra to it,” says Carol Brodie, Winston’s global director of communications.

Larger diamonds, those 4 to 50 carats, are ideal candidates for an emerald cut, which creates a depth illusion much like a swimming pool, Brodie adds. It’s usually a square or rectangle shape, and it looks as if there is a square within a square.

“Desperate Housewives”‘ Teri Hatcher was wearing a 31-carat emerald-cut diamond cuff bracelet when she won her best actress Golden Globe.

The marquise is long and narrow with points at the two vertical ends and is probably the closest to the diamond shape taught in junior high geometry.

The marquise had its heyday in the 1950s and ’60s, Mognaz says, but it’s due for a comeback since it’s a flattering shape that gives length to the hand and fingers. Asscher-cut diamonds, with their high and deep crowns, were most popular in the World War II-era, but they, too, are enjoying a bit of a resurgence, as are pear-shaped gems.

An oval diamond is the newest of the classic shapes, but it’s also the least common.

There are the “four Cs” that shoppers are supposed to keep in mind when sizing up diamonds: color, cut, clarity and carat. “I can’t emphasize cut enough,” Mognaz says.

Faceting is everything, Brodie agrees. “A diamond is just a rock. A genius cutter can see the beauty in that rock. It’s the ultimate extreme makeover.”

The cutting or faceting is what gives the stone its fire and light. “When a diamond is cut to perfect proportions — neither too deep nor too shallow — light will reflect inside the stone from one mirror-like facet to another and disperse through the top of the stone, making it seem to glow from within,” according to the Harry Winston diamond guide.

Setting also is important, notes Mognaz, because too much metal covers up the stone and limits its light.

Both Mognaz and Brodie encourage diamond shoppers to seek out reputable jewelers and to make sure they get the certificate from the Gemological Institute of America, which should come with all new diamonds. (Reputable doesn’t necessarily mean expensive; they name Zales, which sells pendants for less than $100 as a reputable jeweler.)

Shoppers also should educate themselves on how to read the certificate, they add. For example, a diamond rated FL is considered flawless and is therefore the most valuable. An IF diamond is internally flawless, which means it has no “clouds” or “feathers” inside the stone; a VVS diamond has very few of those flaws; a VS has slightly more.